1. Field of the Invention
The present invention relates to a method for establishing a three-dimensional map of the spatial distribution of the lithologic composition of sediments deposited in a sedimentary basin during a given geologic time interval, as well as the temporal evolution of the depositional profile throughout filling of the basin, while respecting exactly the thicknesses of the sedimentary sequences measured otherwise.
FIG. 1 shows three filling stages of a sedimentary basin. The fill of a sedimentary basin has superposed layers (C1, C2 and C3), also referred to as sedimentary sequences, which correspond each to the deposition of sediments during a sub-time interval of the geologic period required for filling of the basin. Each layer is thus limited by a lower surface (referred to as “base” of the sequence) and an upper surface (referred to as “top” of the sequence). Thus, the top of a sequence coincides with the base of the next sequence. The base of the initial sequence corresponds to the bottom of the basin; it is the “basement” of the basin. These surfaces evolve with time, due to the mechanical deformations undergone by certain sedimentary basins, during or after deposition. Thus, the “subsidence” (S) describes the depth point of the basement in relation to an absolute reference mark linked with the globe (the subsidence varies with time) and the eustasy (E) describes the ocean surface variations recorded simultaneously on all of the earth's surface. The “accommodation” (A) is the vertical distance between the basement (basin bottom) and the sealevel, which also varies with time. The “depositional profile” represents the depositional surface of the sediments at any time and is the topographic surface. Finally, the “bathymetry” (b) describes the vertical distance between the depositional profile and the sealevel.
2. Description of the Prior Art
Recent advances in geology, which gave birth during the past twenty years to seismic stratigraphy, then to genetic stratigraphy, have deeply modified understanding of the history of sedimentary filling of sedimentary basins over large time and space scales, by showing the major influence of two main parameters: the temporal evolution of the accommodation and the sediment supply at the basin boundaries.
Many models and notably deterministic numerical models have been developed to allow apprehending the geometric and lithologic implications of these new approaches.
These numerical models simulate transport and sedimentation (or erosion) of the sediments in the basin, on the basis of a more or less complex description of the nature, from an estimation of the eustasy, of the subsidence and of the sediment supply at the boundaries of the basin studied.
Among these numerical models, the diffusive models have proved efficient through their use in many studies carried out notably for the petroleum industry in order to better and more readily delimit zones likely to contain hydrocarbons. Such models are for example described in the documents hereafter:                Rivenaes, J. C., 1988, Application of a Dual-Lithology, Depth Dependent Diffusion Equation in Stratigraphic Simulation, Basin Research, 4, 133-146,        in the patents of the applicant: French Patent 2,744,224 and U.S. Pat. No. 5,844,799,        and in published patent application No. FR-2,849,211.        
The latter two references relate to methods for modelling the filling of sedimentary basins.
These deterministic numerical models are integrated in a procedure for calibrating their input parameters, referred to as “inversion procedure”. This inversion procedure is intended to adjust the parameters of the model so that the results provided thereby best fit the reality observed. The temporal evolution of the subsidence and of the sealevel, that is of the accommodation, is among the parameters to be adjusted. The adjustment criterion for the model obtained is based, among other things, on the capacity of the model to reproduce the geometry, and notably the map of the deposited sedimentary unit thicknesses.
This inversion procedure is in most cases of “trial-and-error” type, as described for example in the aforementioned French Patent 2,849,211. It can also be automated as described, for example, in French Patent 2,776,393 filed by the assignee, which relates to a stratigraphic reservoir modelling method, or in the following publication for example:                T. A. Cross and M. A. Lessenger, Construction and Application of a Stratigraphic Inverse Model, Numerical Experiments in Stratigraphy; Recent Advances in Stratigraphic and Sedimentologic Computer Simulations, Special Publication—Society for Sedimentary Geology. 62; pp. 69-83.1999.        
In the aforementioned methods, implementation of the inversion procedure involves repeated use of the direct model. Furthermore, although these methods improve the agreement between the reality observed and the model obtained, these methods do not ensure a satisfactory agreement between the reality observed and the result of the model.
Notably, the thickness maps of the various sedimentary sequences are generally well constrained in stratigraphic modelling when resultant from interpretation of a seismic survey. Now, exact adjustment of the deposition sequence thickness maps of the model to the maps given by seismic interpretation is absolutely not guaranteed by the inversion procedure. In practice, exact adjustment is nearly never reached. In the case of the trial-and-error type method, the success of the procedure entirely depends on the user's know-how and intuition. This also applies to an automatic procedure and can never succeed if the data initially proposed by the user are too far from the solution.
In response to these major problems of adjustment to the thickness measurements, a modelling procedure based on the stationary diffusion principle is presented in French Patent Application 03/11,194. It provides an approach allowing carrying out a simulation based on a diffusive principle, wherein the sedimentary sequence thickness maps become a datum and not an adjustment parameter. However, this procedure requires a hypothesis according to which the sediments settle at constant rate at any vertical of the sedimentary sequences modelled, which is referred to as stationary diffusion model. This hypothesis may appear to be too restrictive in some situations, notably when the diffusion coefficients characterizing the sedimentary transport efficiency become too small, which is the case in the depositional geometries exhibiting clinoforms, which translate deposition of a sediment as soon as it enters the marine environment, without further reworking.